The involute of a catenary is called a tractrix. One end P of a rod PQ of fixed length L attached to a point Q which moves along the x-axis, the initial position of the rod being on the y-axis and the rod moves in such a way as to be always tangent to the path P. The problem was originally stated in physical terms: A man standing at the origin O holds a rope of length L to which a weight is attached. The man walks to the rightdragging the weight after him. When the man is at Q and the weight is at P, find the path of the weight. The elementary differential equation is \(dx/dy=-\sqrt{L^ 2-y^ 2}/y\). The solution is found to be \(x=f(y)\). This paper shows how to find y as a function of x. Involved in this inversion is an exposition of how to find the cube root of an infinite series and the use of the coefficients of reverted series.